In methods for fabricating semiconductor devices, it is necessary to clean the surfaces of Si (silicon) substrates, in some cases. More specifically, in some cases, it is necessary to remove natural oxide films or foreign substances from the surfaces of Si substrates. For example, foreign substances on the Si surfaces may cause abnormal diffusions during addition of impurities thereto.
For example, according to Japanese Patent Application Laid-Open No. 8-339996 (1996) (Patent Document 1), a SiO2 film (a silicon oxide film) is formed on a Si substrate. Further, a SiOx film (x>2) is formed on the silicon oxide film. Further, an opening portion is formed in the layer-laminated structure constituted by the silicon oxide film and the SiOx film. Along with the process for forming the opening portion, a natural oxide film is formed on the Si substrate in the opening portion. This natural oxide film is removed using hydrofluoric acid. This can provide a cleaned Si surface having been hydrogen-terminated in the opening portion.